TM 5-814-3/AFM 88-11, Volume III
and laterals through pipe joints and older brick manholes with increase in groundwater levels can result in large
infiltration. The amount of water that actually percolates into the groundwater table may be negligible if an area
is occupied by properly guttered buildings and paved areas, or if the subsoil is rich in impervious clay. In other
sandy areas, up to 30 percent of rainfall may quickly percolate and then lift groundwater levels. Infiltration rates
have been measured in submerged sewer pipe. Relatively new pipe with tight joints still displayed infiltrations
at around 1,000 gallons per day per mile, while older pipes leaked to over 40,000 gallons per day per mile.
Sewers built first usually followed the contour of water courses and are often submerged while more recent
sewers are not only tighter, but are usually built at higher elevations as the system has been expanded. In
designing new treatment facilities, allow for infiltration as given in TM 5-814-1/AFM 88-11, volume 1, except
as modified by this design manual. Utilize existing flow records, sewer flow surveys, and examine the correlation
between recorded flows and rainfall data to improve the infiltration estimate. The economic feasibility of
improving the collection system to reduce the rate of infiltration should be considered.
(2) Another method for calculating the infiltration component of total flow is to multiply the miles of
a given pipe size and condition by the diameter in inches and to sum the inch-miles. The sums of inch-miles of
pipe estimated according to conditions are then multiplied by factors between 250 and 500 to obtain gallon/day.
If infiltration is known to be negligible at manholes, then an infiltration allowance may be calculated based upon
area served and figure 4-1. Curve A should be used for worst conditions when pipes are old and joints are
composed of jute or cement. Curve B applies to old pipes with hot or cold asphaltic joints or for new pipes
known to have poor joints. Curve C is used for new sewers where groundwater does not cover inverts and when
joints and manholes are modern and quite tight. Of course, field tests may be conducted to more closely estimate
infiltration.
(3) Average wastewater flow is usually expressed in million gallons per day, but will be calculated in the
appropriate units for design of the unit process under consideration.
c. Contributing populations. In calculating contributing populations, use 3.6 persons per family residen-tial
unit. In hospitals, count the number of beds, plus the number of hospital staff eating three meals at the hospital,
4-3
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